Carbon footprint and sustainability is increasingly important for all of us -- so how does the ecological footprint of your computing habits pan out?
The simplest thing you can do is to make sure that your desktop or laptop goes into sleep or standby mode when it's not in use. First up, make sure that you're not using a screensaver, or there will be little or no energy consumption difference between idle and in use. Next, KDE and Gnome both have menu items where you can set power preferences; or you can of course set things manually with various XF86 options, hdparm, and so on.
Next, consider whether you really need to leave your machine running overnight. If you run cronjobs overnight, try changing the timing to another lull time; or replace cron with anacron. You can also set up automatic shutdown if you tend to forget to turn the box off (I do). Both power management changes and overnight powerdown aren't just the green option, they're also the cheaper option: reduce your power bill...!